A dental implant is an artificial tooth root that periodontists place into the jaw to hold a replacement tooth or to support a replacement prosthesis. Treatment with dental implants is widely accepted and holds a number of advantages over other techniques such as removable partial dentures, bridges or loose prostheses. Dental implants allow reconstruction of the dentition without the need to sacrifice healthy neighbouring teeth. The loads transferred via the implants stimulate the bone, preventing bone resorption and limiting recession of the gums around the replaced tooth elements and resulting in a more aesthetic reconstruction. Treatment with implants also provides a more comfortable and stable solution than conventional dentures, guaranteeing more natural biting and chewing capacity.
Surgical planning for dental implant placement traditionally uses one or more medical imaging modalities such as an orthopantogram (i.e. an x-ray technique for imaging the bones of the jaws and the teeth) or computerized tomography (CT) scan to verify the quantity and quality of the bone. In fact, the American Association of Oral and Maxillofacial Radiologists (AAOMR) recommends that some form of cross-sectional imaging be used for implant treatment. According to the classic way of working, the final decision about the implant positions however is taken during surgery when opening of the surrounding soft tissue has exposed the bone. After an osseointegration period of 3 to 6 months secondary, mainly angular corrections are made to the direction of the implants to optimize the aesthetics of the reconstruction. Still, the design of the final implant-supported prosthesis is dictated to a large extent by the original implant positions, which may be suboptimal from an aesthetical, functional and biomechanical point of view.
The classic solution is no longer valid when evolving towards immediate loading of the implant. Immediate loading of the implants requires a flawless planning and precise surgical transfer. This planning preferably, besides being biomechanically sound, already takes into account aesthetic and/or functional considerations, which using traditional implantlogy methods were only of importance during the actual prosthetic phase of the implant treatment.
Over the last few years several tools have been made available commercially to provide periodontists with a means to evaluate the bone of the patient in a number of differently oriented slices of a volumetric scan, such as a CT scan or other volumetric scans such as MRI and to graphically superimpose on the images representations of commercial implants of varying length, diameter and brand (see SimPlant™ provided by Materialise, Leuven, Belgium). Citing Benjamin, “Multi-planar reformatted CT, has become the most comprehensive and accurate aid for implant treatment planning” (see Benjamin L S, The evolution of multiplanar diagnostic imaging: predictable transfer of preoperative analysis to the surgical site. J Oral Implantol. 2002; 28(3): 135-44).
According to the current state of the art, treatment of a patient with dental implants consists of a number of steps. Before treatment, articulated stone models are firstly used to assess vertical dimension. Afterwards a diagnostic wax-up is created to represent the desired prosthetic end result (see FIG. 1). The wax-up is optimized to achieve proper occlusion, morphology, aesthetics and phonetics. In a next step a scanning template or scan prosthesis is manufactured (see FIG. 2). This is an exact replica of the wax-up made in a radiopaque material, typically a cold-polymerizing resin mixed with a certain percentage of barium sulfate. The level of opacity of the scan prosthesis may vary for its constituent parts: for instance the teeth may have a higher opacity than the base plate. If desired, some parts can be made radio-transparent. When the patient is scanned with the scan prosthesis in the mouth, the radiopaque parts will be clearly visible in the CT images (see FIG. 3). In some cases the main axis of each restorative element e.g. tooth is marked by drilling a cylindrical shaft. Incorporation of the scanning template in the CT images enhances the surgeon's ability to plan in function of the desired prosthetic outcome.
Following production of the scan template, the patient is sent to a radiologist for a CT scan. The scan template is placed in the patient's mouth and the scan is taken. The output of the scan is a stack of 2D slices forming a three-dimensional “data set”.
Once the CT scan has been taken and the 3D models constructed (see FIG. 4), the surgeon plans the implant treatment using a computer program. Typically, such a program imports the data set provided by the radiology site without altering any information. Using image-processing techniques (e.g. image segmentation) three-dimensional models of the bone are derived from the data set. Given that the radiopaque dentition is well represented in the 2D axial slices, a 3D model of the desired prosthetic set-up can also be constructed.
Instead of using a radiopaque scan prosthesis, sometimes the diagnostic wax-up or a loose prosthesis is digitized separately (via CT, optical scanning or mechanical scanning) and afterwards registered to the anatomical structures visible in the volume data. This way information about the desired dentition is also obtained, in its correct relation relative to the jaw. The computer program (see FIG. 5) allows the individual patient's CT images to be assessed in a three-dimensional way and to determine where dental implants can be placed ideally. Implants can be chosen from a digital implant library (different implant brands, lengths, diameters, etc.).
The practitioner next defines a panoramic curve in the axial images (see FIG. 6). The curve typically follows the arch of the jaw. Several cross sections (see FIG. 7) can be chosen perpendicular to both the panoramic curve and the axial slices. Typically implant receptor sites are chosen in these cross sections. The practitioner can modify the positions and inclinations of each implant as needed in any of the views (axial, panoramic, 3D or cross sectional). Fine tuning is done by shifting and tilting of the implant representations or by changing their dimensions. Each individual implant position can be evaluated in terms of the volume of available bone, described as the “triangle of bone” by Ganz (Ganz S D, The triangle of bone—A formula for successful Implant Placement and Restoration, The implant society, Inc. 1995 Vol. (5); 5 pp 2-6). The quality of the bone is visualized in the computer program using, for example, Hounsfield units as a measure for bone density.
Once the implant plan has been fixed, the surgeon must transfer it as accurately as possible to the patient. This transfer can be done mentally, using custom made guiding templates, e.g. as supplied by Materialise, Leuven Belgium under the name SurgiGuide™ (see FIG. 8) or using alternative means of navigation.
Although the current computer programs for dental implant planning all visualize the information necessary to simulate different implant treatments and all offer a range evaluation tools, none provide automated or semi-automated assistance in determining the optimal position of the implants from a biomechanical, functional or aesthetic point of view.